In cell biology, an organelle is a specialized structure within a cell that performs a specific function and that generally is considered to be separately enclosed within its own lipid membrane.

For example, the nucleus of a cell is an organelle that maintains the integrity of genes and controls cell activity by regulating gene expression. Lysosomes are organelles that digest food particles, worn out organelles, and viruses and bacteria. Mitochondria generate most of a cell's supply of ATP, among other functions. Among other structures known as organelles are vacuoles, peroxisomes, and chloroplasts.

Some subcellular structures commonly called organelles, such as ribosomes, are not actually an organelle under the more restrictive definition of being enclosed within a separate membrane. Ribosomes, being strictly particulate, do not include such a membrane. However, the term organelle sometimes is used in the less restrictive sense of any distinct, subcellular structural unit. Thus, ribosomes are at times described as "non-membranous organelles" or the nucleolus as a "nuclear membraneless organelle."

The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Just as organs provide a particular function for the body, organelles provide a particular function for cells. This harmony is seen on each level of an organism, as cells work together as part of tissues, tissues as part of organs, and organs as part of organ systems. An analogy might be made with a harmonious human society, whereby individuals contribute to their families (as organelles to cells), their families to their communities (as cells to tissues), communities to societies (as tissues to organs), societies to their nations (as organs to organ systems), and their nations to the world (as organ systems to the body), and in turn each would be benefited by those larger entities.

There are many types of organelles, particularly in the eukaryotic cells of higher organisms. Prokaryotes were once thought not to have organelles, but some examples have now been identified (Kerfeld et al. 2005).

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History and terminology

In biology, an organ is defined as a confined functional unit within an organism that performs a specific function or group of functions. The analogy of bodily organs to microscopic cellular substructures is obvious, as from even early works the authors of respective textbooks rarely elaborated on the distinction between the two.

Credited as the first to use a diminutive of organ for respective cellular structures was German zoologist Karl August Möbius (1884), who used the term "organula" (Möbius 1884; Bütschli 1888). Organula is the plural form of organulum, the diminutive of Latin organum. From the context, it is clear that he referred to reproduction related structures of protists. In a footnote, which was published as a correction in the next issue of the journal, he justified his suggestion to call organs of unicellular organisms "organella" since they are only differently formed parts of one cell, in contrast to multicellular organs of multicellular organisms. Thus, the original definition was limited to structures of unicellular organisms.

It would take several years before organulum, or the later term organelle, became accepted and expanded in meaning to include subcellular structures in multicellular organisms. Books around 1900 from Valentin Häcker (1899), Edmund Wilson (1900), and Oscar Hertwig (1906) still referred to cellular "organs." Later, both terms came to be used side by side. Bengt Lidforss wrote 1915 (in German), about "organs or organells" (Lidforss 1915).

Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex," that is, flagella and their anchoring) (Kofoid and Swezy 1919) and other protist structures, such as ciliates (Hamburger 1919). Alfred Kühn wrote about centrioles as division organelles, although he stated that, for Vahlkampfias, the alternative "organelle" or "product of structural build-up" had not yet been decided, without explaining the difference between the alternatives (Kühn 1920).

In his 1953 textbook, Max Hartmann used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists (Hartmann 1953).

Later, the now-widely-used definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles (Nultsch 2001; Wehner et al. 1995; Alberts et al. 2002; Madigan et al. 2001). However, the more original definition of subcellular functional unit in general still coexists (Strasburger and Sitte 2002; Alliegro et al. 2006).

In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli (Frey-Wysling 1978a, 1978b). This new definition, however, did not win wide recognition.

Examples of organelles

Whereas most cell biologists consider the term organelle to be synonymous with "cell compartment," some cell biologists choose to limit the term organelle to include only those that are DNA-containing, assumed to have originated from formerly-autonomous microscopic organisms acquired via endosymbiosis. The most notable of these organelles considered to have originated from endosymbiont bacteria are:

Other organelles are also suggested to have endosymbiotic origins, notably the flagellum.

The use of the term to refer to some subcellular structures is disputed. These structures are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Such cell structures, which are not formally organelles, include:

Eukaryotic organelles

Eukaryotes are the most structurally complex cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the nucleus and vacuoles, are easily visible with the light microscope. They were among the first biological discoveries made after the invention of the microscope.

Not all eukaryotic cells have every one of the organelles listed below. Exceptional organisms have cells that do not include some organelles that otherwise might be considered universal to eukaryotes (such as mitochondria) (Fahey et al. 1984). There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g., some that are listed as double-membrane are sometimes found with single or triple membranes). In addition, the number of individual organelles of each type found in a given cell varies depending upon the function of that cell.

Mitochondria and chloroplasts, which have double-membranes and their own DNA, are believed to have originated from symbiotic prokaryotic organisms (perhaps originally incompletely consumed or invading in character), which were adopted as a part of the host cell. This idea is supported in the endosymbiotic theory.

Minor eukaryotic organelles and cell components

Organelle/Macromolecule

Main function

Structure

Organisms

acrosome

helps spermatoza fuse with ovum

single-membrane compartment

many animals

autophagosome

vesicle that sequesters cytoplasmic material and organelles for degradation

Prokaryotic organelles

Prokaryotes are not as structurally or metabolically complex as eukaryotes, and were once thought not to have any internal structures enclosed by lipid membranes. In the past, they were often viewed as having little internal organization; but, slowly, details are emerging about prokaryotic internal structures.

An early false turn was the idea developed in the 1970s that bacteria might contain membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy (Ryter 1988).

However, more recent research has revealed that at least some prokaryotes have microcompartments, which are compartments enclosed by proteins (Kerfeld et al. 2005). Even more striking is the description of magnetosomes (Komeili et al. 2006; Scheffel et al. 2006), as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes (Fuerst 2005).

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